The present invention relates to the casting molten metals and alloys in shell molds and, more particularly, to fugitive patterns for use in forming the shell molds.
In the well known xe2x80x9clost waxxe2x80x9d process of investment casting, a fugitive or disposable wax pattern is made by injection molding melted wax in a die corresponding to the configuration of the component to be cast. The molded wax pattern then is invested in a ceramic shell mold by repeatedly dipping the pattern in a ceramic slurry, draining excess slurry, and stuccoing with coarse ceramic particles or stucco until a desired thickness of a ceramic shell mold is built-up on the pattern. The pattern then is removed from the green shell mold typically by heating the shell mold to melt out the pattern, leaving a ceramic shell mold which then is fired at elevated temperature to develop appropriate mold strength for casting a molten metal or alloy.
Attempts have been made to form fugitive patterns from polystyrene or polyurethane foam for practice of the xe2x80x9clost waxxe2x80x9d process to make ceramic shell molds. However, polystyrene or polyurethane foam patterns have not been used in high volume commercial production of xe2x80x9clost waxxe2x80x9d casting due to certain shortcomings of the patterns with respect to pattern surface quality, pattern strength, and difficulty in removal of the pattern from a shell mold invested therearound without cracking of the shell mold. For example expired U.S. Pat. No. 3,747,663 found that polystyrene or polyurethane foam patterns have easily damaged surfaces with a non-smooth, open cell surface structure. The patent coats the pattern with a film-forming polymer in a separate operation prior to being invested in a ceramic shell in order to overcome these problems.
An object of the invention is to provide a method of making a shell mold using a reaction injected molded polyurethane foam pattern as well as mold/pattern so formed wherein the pattern is formulated to have pattern surface, strength and other features that render the pattern suitable for making shell molds using xe2x80x9clost waxxe2x80x9d process principles for high volume commercial production of castings without the need to coat the pattern with surface films or layers.
The present invention provides a method of making a shell mold for casting molten metals or alloys by forming a thermally collapsible, low density reaction injection molded (RIM) thermosetting polyurethane foam pattern having a shape corresponding to the casting to be made. The pattern is formulated preferably to have an aggregate pattern density in the range of about 10 to 15 lbs/ft3 (pounds per cubic feet) to provide stiffness or rigidity sufficient to be invested in a shell mold without damage to the pattern yet replication of complex die features, a smooth continuous solid as-molded outer pattern skin or surface devoid of surface connected open cells from the underlying pattern internal microcellular core structure, dimensional stability over a range of temperatures, removability by ashless burnout from the shell mold formed thereon without cracking the shell mold. The pattern is free of organometallic catalysts that should not be present in the casting of aerospace superalloys, such as nickel and cobalt base superalloys. The pattern then is invested in a ceramic or refractory shell mold without the need for any surface polymer or other film on the pattern.
In an embodiment of the invention, the RIM polyurethane foam pattern is formed in a one step reaction injection molding operation using a polyol stream with particular additives and a prepolymer isocyanate stream that form polyurethane foam in an injection mold having die cavity shaped to correspond to the desired pattern shape. The polyol stream includes selected additives such as organic catalysts for controlling gelling and cross-linking, water blowing agent, and surfactant, that cooperate to produce a molded aggregate pattern density of about 10 to about 15 lbs/ft3 and smooth pattern skin or surface.
In a particular embodiment of the invention, the pattern formulation consists essentially of, in parts by weight (pbw) of the formulation, a high molecular weight polyether polyol in an amount of about 20 to about 50 pbw, a lower molecular weight polyether polyol in an amount of about 50 to about 80 pbw, diamine skin-forming additive that assists in producing the smooth continuous defect-free pattern surface in an amount of about 5 to about 15 pbw, a chain extender in an amount of about 2.5 to about 10 pbw, water blowing agent in an amount of about 1 to about 4 pbw, non-silicone surfactant in an amount of about 1 to about 4 pbw, tertiary amine catalyst and amine catalyst in respective amounts of about 0.1 to about 1.0 pbw and about 0.05 to about 0.2 pbw to control blowing reaction, cross-linking and gelling catalyst in an amount of about 0.015 to about 0.075 pbw, and diisocyanate in an amount of about 79.23 to about 190.96 pbw with an isocyanate index of 102 to 105.
Objects and advantages of the invention will become more readily apparent from the following detailed description.